Normal aging is accompanied by a number of cognitive changes, including decline in working memory and executive function/attention control, as well as in long-term memory and speed of processing. These normal age-related declines in cognitive functions are exacerbated in those older adults who suffer from mild cognitive impairment (MCI), which, according to the American College of Physicians, affects about 20% of the population over 70 years old. Beyond this class of individuals suffering from MCI, some among the older population develop Alzheimer's disease or other forms of dementia. An estimated 26.6 million people worldwide had Alzheimer's in 2006 and this number may quadruple by 2050. Increasing knowledge about possible predictors of age-related cognitive impairments and their interactions, providing means of early detection, and improving the chances of successful prevention and/or treatment of the impairments can yield substantial societal and economic benefits.
Recent work with human and animal subjects suggests that cardiopulmonary (aerobic) fitness is one such predictor, and that increased level of fitness can slow down or even reverse some of the cognitive effects of aging. For example, highly-fit older adults were shown to have more preserved white and grey matter in brain areas susceptible to age-related loss, such as frontal, parietal, and temporal cortex. The same older adults demonstrate increased performance in neuropsychological and cognitive tasks compared to age and gender-matched low-fit older adults. Highly-fit older adults also display patterns of brain function that are similar to those of younger adults. Finally, sedentary older adults who undergo a 6-month exercise intervention have been shown to improve in all of these aspects of cognitive health—brain anatomy, function and behavioral measure of performance. Measurements of brain function provide invaluable information for these purposes. Cerebrovascular function is particularly informative in this respect and can be viewed as a complex phenomenon in which different cerebrovascular compartment, such as large arteries, regional arteries, arterioles and capillaries, play interdependent roles.
One measure of the cerebrovascular health is stiffness or, alternatively, elasticity of arteries in the brain. Arterial stiffness may be correlated with cognitive aging as well. Generally, data representative of arterial stiffness may be obtained through measurements of pulse in a given artery. Therefore, imaging of the brain can be performed to obtain spatial brain maps containing data that represent arterial stiffness in order to facilitate the understanding of different cerebrovascular compartments, different aspects of brain anatomy and function, and complex profiling of the cognitive function of the brain.
Traditionally, studies of brain function in human subjects rely on surface measurements of electrical potentials, such as electroencephalogram, or EEG, ultrasound, functional magnetic resonance imaging (fMRI), or various tomographic techniques such as computer X-ray tomography (CT) or positron emission tomography (PET). All these techniques are powerful and have some advantages, but they suffer from drawbacks that limit their use as continuous, non-invasive, portable, and low-cost medical monitors. For example, EEG can be inaccurate and does not lend itself to creating accurate spatial maps. On the other hand, while PET and fMRI lend themselves to providing useful spatial maps of changes in brain functions, such as metabolism or blood flow, fMRI requires high-energy magnetic fields and PET data cannot be obtained quickly enough for informative comparison with other type of data. Furthermore, PET requires the use of a radionuclide or radiopharmaceutical, which can be undesirable. Likewise, CT requires the use of an ionizing radiation, which can also be undesirable.
Conventional non-invasive methods of characterizing the brain arteries, specifically, blood-flow velocity and direction, include transcranial Doppler (TCD) ultrasonography. However, ultrasound-based methods, such as TCD, produce only localized images and do not lend themselves to spatial mapping. Furthermore, TCD is often disfavored in clinical settings due to the level of interference presented by the skull bones that significantly deform and disperse the propagation of the ultrasound waves thus making it complicated to provide localized estimates of intracranial structures. Rather, when utilizing ultrasound to study arterial stiffness, the ultrasound analysis methods are typically limited to localized studies of arteries providing blood flow to the brain, such as arteries in the neck.
It is desirable, therefore, to have a system and method for studying the performance of brain blood vessels, including vessel stiffness, for the purpose of characterization of vascular health of the brain that does not suffer from the above-described drawbacks of traditional imaging systems.